An image sensing apparatus such as a digital still camera or the like, which comprises an electronic viewfinder (to be abbreviated as an EVF hereinafter) that displays the output from an image sensing element on a display device to confirm the composition and exposure upon actual image sensing as a viewfinder, has been proposed.
Also, in an image sensing apparatus such as a digital still camera or the like, a method of computing the exposure level by measuring the object luminance on the basis of a sensed image signal output from an image sensing element has been conventionally proposed.
As photometry methods of these image sensing apparatuses to measure the object luminance, center-weighted photometry that computes a target luminance by weighting with predetermined coefficients using the central region of a screen as the center on the basis of a sensed image signal of a specific region of the screen or sensed image signals of a plurality of divided regions, and evaluative photometry that computes a target luminance using a specific algorithm on the basis of information of each region are known.
Also, an image sensing apparatus which comprises an exposure correction function of correcting the exposure value in accordance with the photographer's will irrespective of the photometry method is generally known.
An example will be explained below.
FIG. 11 is a block diagram of a digital still camera.
Referring to FIG. 11, reference numeral 1 denotes an optical lens for forming an optical image on an image sensing element. The optical lens 1 includes a focus lens (not shown) for focus adjustment. Reference numeral 2 denotes a stop & shutter which has both stop and shutter functions; and 3, a driving circuit for mechanical units of the optical lens 1 and stop & shutter 2. Reference numeral 4 denotes an image sensing element for converting an object image formed by the optical lens into an electrical signal; 5, a timing signal generation circuit (to be abbreviated as TG hereinafter) for generating timing signals required for operating the image sensing element; 6, an image sensing element driving circuit for amplifying a signal from the TG to a level at which the image sensing element can be driven; 7, a pre-processing circuit which comprises a CDS circuit and amplifier circuit used to reduce output noise of the image sensing element 4; 8, an A/D converter; 9, a sensed image signal processing circuit; 10, a recording medium which comprises, e.g., a memory card that complies with PCMCIA standards, hard disk, or the like; and 11, an interface circuit for recording a signal on the recording medium 10. Reference numeral 12 denotes a system controller serving as a control CPU for controlling the respective mechanisms, a console, and the sensed image signal processing circuit; 13, a console used to externally control the camera; 14, a display signal processing circuit for displaying a signal on a display device; 15, a D/A converter; and 16, a display device used as a viewfinder.
In the image sensing apparatus shown in FIG. 11, when the photographer operates the console 13 to start viewfinder display, the mechanism driving circuit 3 is driven under the control of the system controller 12 to supply electric power to respective image sensing circuits. The stop & mechanical shutter 2 is driven to a predetermined default aperture size to start exposure of the image sensing element, and electronic shutter and read pulses are supplied from the TG 5 to the image sensing element 4 via the image sensing element driving circuit 6 so as to accumulate a photocharge for only a predetermined default exposure time. The signal read in the aforementioned state is converted into a luminance level signal for photometry and a viewfinder output signal by the sensed image signal processing circuit 9 via the pre-processing circuit 7 and A/D converter 8. The luminance level signal is sent to the system controller 12, which determines an exposure value corresponding to the luminance level, computes an aperture value and shutter speed in correspondence with the exposure value, and controls the stop & mechanical shutter 2 and electronic shutter for the next exposure in accordance with the obtained values. After that, photometry is repeated in units of fields to control exposure. The viewfinder output signal is sent to the display device 16 via the D/A converter 15 and display signal processing circuit 14. The display device 16 displays an object which is being sensed.
Photometry of Each Frame
The operation inside the system controller upon photometry will be explained below. In FIG. 11, the sensed image signal processing circuit 9 sends, to the system controller 12, luminance values YA, YB, and YC respectively obtained by integrating signals on screen regions A, B, and C shown in FIG. 12.
The photometry process in the system controller will be explained below. Let Sa, Sb, and Sc be the areas of the screen regions A, B, and C. Then, luminance values Ya, Yb, and Yc per unit area of the screen regions A, B, and C are respectively given by:Ya=YA/Sa Yb=YB/Sb Yc=YC/Sc 
By computing the logarithms of differences from a predetermined luminance level reference value Yref, differences from appropriate luminance levels of the respective frames are respectively obtained as dEv—a, dEv—b, and dEv—c:dEv—a=log2(Ya/Yref)dEv—b=log2(Yb/Yref)dEv—c=log2(Yc/Yref)
On the other hand, let Av0 be an Av value obtained by converting the aperture size set in the stop & mechanical shutter 2 upon exposure, and Tv0 be a Tv value obtained by converting an exposure time. Then, Ev0 as a set Ev value upon exposure is given by:Ev0=Av0+Tv0
The Ev values of the respective frames are respectively given by:Ev—a=Ev0+dEv—a Ev—b=Ev0+dEv—b Ev—c=Ev0+dEv—cAveraging Photometry
Let Ev1 be the luminance level of the entire screen, which is obtained by weighting the regions A, B, and C in accordance with their areas, and is given by:Ev1=Ev0+(dEv—a×Sa+dEv—b×Sb+dEv—c×Sc)/(Sa+Sb+Sc)Center-weighted Photometry
On the other hand, let Ev2 be the luminance level of the entire screen obtained by weighting the regions A, B, and C using predetermined weighting coefficients kWeiA, kWeiB, and kWeiC as per:Ev2=Ev0+(dEv—a×kWeiA+dEv—b×kWeiB+dEv—c×kWeiC)/(kWeiA+kWeiB+kWeiC)Note that weighting coefficients kWeiA, kWeiB, and kWeiC are set to satisfy:(kWeiA/Sa)>(kWeiB/Sb)>(kWeiC/Sc)so as to achieve center-weighed photometry.Evaluative Photometry
The luminance differences between the regions A and B, and B and C are given by:deltaBA=Ev—b−Ev-a deltaCB=Ev—c−Ev—b An exposure correction value α of evaluative photometry is computed from the values deltaBA and deltaCB. For example, it is determined that the luminance of the screen central portion is lower, i.e., the degree of back light is larger with increasing values deltaBA and deltaCB, and α is increased to attain back light correction.
Let Ev3:Ev3=Ev2−αbe the target luminance level obtained by correcting α.
Using Ev1, Ev2, and Ev3 as exposure target values in the averaging, center-weighted, and evaluative photometry modes, respectively, various photometry schemes can be provided.
Exposure Correction
When Compβ represents an exposure correction value set when the photographer operates the console 13, and Ev4, Ev5, and Ev6 represent the sums of the exposure target values Ev1, Ev2, and Ev3Ev4=Ev1+Compβ (averaging photometry)Ev5=Ev2+Compβ (center-weighted photometry)Ev6=Ev3+Compβ (evaluative photometry)in the respective photometry mode and Compβ, i.e., exposure target values upon setting exposure correction, an image sensing apparatus with an exposure correction function can be provided.AE During EVF Display
In an EVF display mode, photometry for obtaining an exposure target value and exposure as image sensing for EVF display must be simultaneously done.
Hence, when Ev0 as the luminance value upon exposure adopts an exposure target value computed upon exposure one or more cycles before, an exposure result corresponding to the selected photometry mode and exposure correction value can always be confirmed on the EVF.
As a versatile image sensing element used in the image sensing apparatus such as a digital still camera or the like, a CCD is known. The CCD can measure a luminance range of around 8 EV.
Since the luminance range of a general object is around 5 EV, the luminance of the object can be accurately measured by setting the object central luminance as the center of the luminance distribution of the object to be close to the photometry central luminance as the center of the luminance range measured by the CCD.
In a still camera with an EVF, by displaying an image sensed using an exposure value equivalent to the exposure target value upon actual image sensing on the EVF, whether or not exposure of the image is appropriate can be estimated in advance. For this purpose, in the EVF display mode, photometry for computing an exposure target value and exposure for sensing an EVF display image are simultaneously done. Hence, the exposure target value inevitably matches the photometry central luminance in this case.
However, when evaluative photometry or exposure correction is done, as described above, the exposure target value does not often match the object central luminance, and has a difference of several EV in some cases. That is, since the photometry central luminance is separated from the object central luminance, the object luminance range readily falls outside the photometry luminance range of the CCD, and it becomes impossible to measure the object luminance in this case.
When photometry is made using the object central luminance or an exposure target value obtained by averaging photometry irrespective of the exposure target value, the luminance distribution of the object is highly likely to fall within the dynamic range of the CCD. However, the exposure result using the exposure target value of the selected photometry mode cannot be confirmed on the EVF in advance.